In recent years, programmable controllers have become quite popular in controlling various industrial processes. These controllers generally include a central processing unit which is programmed to perform a series of steps which include controlling the condition of various output terminals and reading the output conditions for subsequent processing. Generally, the output signals and the input conditions from output circuits are directed through a seaprrate module. Since a large number of output units are required in most situations, the capabilities of a single module are exceeded. Thus, except for very simple applications, a series of modules for controlling the various outputs are required. These modules are generally referred to as input/output or I/O modules, since they are capable of both setting an output and reading the condition of the various outputs controlled by the individul modules. The need for a plurality of output modules to accommodate the large number of output units dictates that a programmable controller system must include a number of individual I/O modules. Thus, in order to set a selected output and/or read the selected output, the particular module connected to selected outputs must be addressed by the programmable controller. This is generally done by decoding four address lines to produce one of sixteen separate codes on sixteen parallel lines. In this manner, up to sixteen individual modules can be used for receiving from and transmitting information to the various output units. In the past, each of the individual modules was assigned a number and the module of a selected number was provided with a decoding network of logic gates so that an enable signal on one of the parallel lines or conductors would actuate a selected one of the I/O modules. This general arrangement has certain disadvantages. Each of the modules must be provided with a selected, different decoding network so that two or more modules will not be enabled simultaneously. Such decoding networks also increase the time to transmit data through the modules. In addition, the particular decoded number of a module must be marked on the module or otherwise recorded so that an assembler of the system will know which module is used for controlling which connected output units.